import java.util.*;

class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode() {}
     TreeNode(int val) { this.val = val; }
     TreeNode(int val, TreeNode left, TreeNode right) {
         this.val = val;
         this.left = left;
         this.right = right;
     }
 }


public class LeetCode {

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root == null){
            return ret;
        }
        Queue<TreeNode> q = new ArrayDeque<>();
        q.add(root);
        int level = 1;
        while(!q.isEmpty()){
            int size = q.size();
            List<Integer> tmp = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode t = q.poll();
                tmp.add(t.val);
                if(t.left!=null){
                    q.add(t.left);
                }
                if(t.right!=null){
                    q.add(t.right);
                }

            }
            if(level%2==0){
                Collections.reverse(tmp);
            }
            ret.add(tmp);
            level++;
        }
        return ret;
    }
}
